1. Field of the Invention
This invention relates to a torque converter with a lockup mechanism for an automatic transmission for vehicles. More particularly, the invention relates to the cooling mechanism of a torque converter having a lockup mechanism in it, which is capable of cooling the frictional surface of a lockup (directly coupled) clutch, and of controlling slippage at the same time when the lockup clutch slips between its joined (or engaged) and released conditions.
2. Related Background Art
In general, with a torque converter, smooth driving can be realized since the power is transmitted through a fluid. On the other hand, it has a drawback that mileage is lowered due to the energy loss resulting from the slippage of the fluid. In order to eliminate this drawback, recent torque converters are provided with a lockup mechanism.
The lockup mechanism comprises a lockup clutch having a frictional surface. The mechanism is arranged so that the flow of a fluid in the torque converter is automatically changed when the traveling speed of a vehicle becomes higher than a predetermined speed, and then, the frictional surface of the lockup clutch is pressed to the front cover of the torque converter to directly couple the engine with the driving wheel. In this way, the lockup mechanism can eliminate the influence exerted by the fluidal slippage, hence attaining the enhancement of mileage.
The lockup clutch is displaced between the joined and released conditions like a clutch which is used with a usual manual transmission. Between the joined and released conditions of a lockup mechanism, there is provided a slipping state where the frictional surface is temporarily caused to rotate while correlatively sliding (corresponding to the half-clutched condition of a clutch used for a manual transmission) in order to utilize the friction force for softening the shock to be exerted or enhancing the transmission efficiency of the torque converter when it is in the lockup state.
In the slipping state, where the lockup clutch slides, the friction force is generated on the frictional surface and the front cover of the torque converter, thus causing excessive heat to be generated. There is a possibility of its resultant accumulation.
Usually, therefore, the torque converter having a lockup mechanism in it is provided with a cooling mechanism to remove frictional heat. For example, in the specification of U.S. Pat. No. 5,209,330, there is disclosed a torque converter having oil grooves for cooling which are open to enable a fluid to pass between a lockup clutch and the plate surface of its counterpart even when the lockup clutch is in a completely joined state. In other words, the converter is provided with a frictional member for the lockup clutch, which is not sealed (not closely in contact).
Also, in the specification of U.S. Pat. No. 5,215,173, there is disclosed a torque converter having a frictional member which is totally sealed in a state where the lockup clutch is completely joined. In this U.S. patent, it is also disclosed that this lockup clutch is cooled from the back side of the clutch piston to which the frictional member is adhesively bonded.
However, for the torque converters disclosed in the two U.S. patents described above, and also, those disclosed by the prior art, there are problems yet to be solved as described below.
In torque converters according to the prior art described above, the difference in pressure is balanced with respect to those exerted between the front cover of the torque converter and the clutch piston, and between the clutch piston and the main body of the torque converter in order to make it possible to displace the lockup clutch between its joined and released conditions. Consequently, the structure is arranged in order to make the required difference between those pressures constant for the maintenance of the slipping state of the lockup clutch in a position between its joined and released conditions.
Because of this structural arrangement, the aforesaid difference in pressure tends to vary if the flow rate of a fluid supplied to the frictional surface should be increased in order to suppress the excessive frictional heat generated particularly when the lockup clutch is in the slipping state. In other words, if the flow rate of the fluid supplied to the frictional surface should be increased or decreased, the difference in pressure is also increased or decreased almost in proportion thereto.
Then there is a possibility that the slipping state cannot be maintained. As a result, a drawback is encountered that a positive supply of the fluid is not allowable even when it is needed to suppress the frictional heat while the lockup clutch is in a slipping stat.